Targets netstandard1.0 and net45 and updates build infrastructure to latest .NET Core SDK

[cwe1ss]

This PR does two things:

• Builds now require the latest .NET Core SDK (2.1.4). This is enforced in the new global.json
  • dotnet restore is now an implicit operation in dotnet build so it's no longer required
• The OpenTracing library now also targets "net45" which was requested in Target both netstandard1.0 and net45 to remove unneeded dependencies #51. The code in this PR doesn't require a separate msbuild parameter though.

Since this doesn't contain any functional changes, I will only wait until Monday or Tuesday to merge it -
unless there's contrary feedback of course.